(a) Field of the Invention
The invention relates to a television tuner, and particularly to a tuner for digital television.
(b) Description of the Related Art
An analog television is easily influenced by terrain or landscape which brings about poor signal receiving and large signal amplitude fluctuation. A digital television (hereinafter referred to as “digital TV”) that overcomes the cons of an analog television not only offers high quality image and audio but also more programs while using the same amount of bandwidth. In addition, the digital TV also provides functions supporting data broadcasting such that various services and interactivities are derived from the functions. However, drawbacks such as slow response speed and residual image during channel switching are still unacceptable to the general public for they are perceivable to human vision. Therefore, the popularity of the digital TV is yet to be promoted.
FIG. 1 illustrates a conventional digital TV tuner 10. The digital TV tuner 10 receives a digital radio frequency signal Rf while a signal is sent to a control unit CS according to the user command through a remote controller, a channel tuner or the like. The control unit CS generates a channel control signal C to select the signal according to the channel requested by the user. Then, the target image data T of the requested channel is generated after the signal is processed through filtering, demodulating, decoding. The control unit CS can be a central processing unit, CPU. The digital TV tuner 10 comprises a control unit CS, a RF tuner 11, a signal filter 12, a DTV IF demodulator 13, and a DTV image decoder 14. The signal filter 12 can be a surface acoustic wave (SAW) filter. The DTV image decoder 14 can be a digital TV MPEG decoder. The RF tuner 11 receives a digital radio frequency signal Rf and extracts a signal from the digital radio frequency signal Rf for the channel requested by the user according to the channel control signal C. The signal of that channel is thereby converted into a digital intermediate frequency signal If. The signal filter 12 receives the digital intermediate frequency signal If and generates a filtered intermediate frequency signal If′. Then, the DTV IF demodulator 13 of the digital TV converts the filtered intermediate frequency signal If′ into a low frequency image compression data Ic. The DTV image decoder 14 then decodes the image compression data Ic into a target image data T and transmits the target image data T to a display device (not shown in the figure) for viewing.
At present, the image data format of the digital TV is generally chosen from specifications such as MPEG2, MPEG4, H.264, . . . and so forth. Assuming that the tuner 10 of the digital television is designed according to the MPEG standard, then the image data format is like what is shown in FIG. 2A. The image data includes I frames, P frames, and B frames. The I frames are scene image data and if one I frame switches to another I frame, the scene changes. The P frames are image data of scene translations while the B frames are image data regarding motions other than the scene. Generally, the MPEG encoded image data is transmitted with a certain sequence during transmission and decompression. As shown in FIG. 2A, when the user switches to a channel, the DTV IF demodulator 13 locks on the frequency band of that channel and converts the filtered intermediate frequency signal If′ into a MPEG encoded image compression data Ic. Then, the DTV image decoder 14 decodes the first I frame i1 emerged from the image compression data Ic. The DTV image decoder 14 then receives the first emerged P frame p1. Finally, the DTV image decoder 14 predicts the B frame b1 based on the I frame i1 and the P frame p1 to process the image of that channel step by step.
When the user switches to another channel, the DTV IF demodulator 13 must lock onto the bandwidth of another channel and the DTV image decoder 14 then decodes the first emerged I frame. As the MPEG image data is transmitted with a certain sequence during transmission, the DTV image decoder 14 may or may not immediately receive the I frame of the switched channel and may receive the P frame instead. In the case of receiving the I frame immediately, the user will be able to see the image of the requested channel right away. However, if the P frame is received first, the user will have to wait a few more seconds until the I frame is received in order to display the entire image of the requested channel. Since it is more probable to receive a P frame first rather than an I frame, a delay will therefore appear when the user switches channels. As shown in FIG. 2B, when the user switches to channel 2, channel 3, and channel 4 at time t1, t2, and t3, the I frames of these channels appear at a later time of t1, t2, and t3, and thus experiences a serious delay of the images to be displayed when the user switches channels.
The inconvenience of the delay phenomenon while switching between digital TV channels has affected the promotion of digital TV. Hence, a digital TV tuner to reduce the time required for the re-locking and decoding and to achieve fast channel switching is yet to be provided.